1. Field of the Invention
The present inventions relates to a variable time-constant type differentiator adapted for use with an active filter circuit comprising multistage-connected operational amplifiers or the like.
2. Description of the Prior Art
Referring to FIG. 1 of the accompanying drawings, a conventional differentiator is illustrated wherein a resistor R.sub.o is connected across the input and output terminals of an operational amplifier A.sub.o to which a signal is inputted through a capacitor C.sub.o. To achieve a variable time-constant type differentiator, it is required that the resistor Ro be a variable resistor. However, when it is attempted to construct, by using such differentiators, a filter circuit in the form of a semiconductor integrated circuit which is connected in multistage configuration like an active filter, especially a low pass filter, it is demanded that external resistors or the like be eliminated to a maximum possible extent to achieve one-chip construction and miniaturization of the circuit.
The foregoing conventional differentiator comprises a resistor, a capacitor and an operational amplifier. Thus, to provide such a differentiator as a variable time-constant type one, an external variable resistor must be employed. Obviously, however, this is undesirable for formation of a semiconductor integrated circuit.
A differentiator in the form of a semiconductor integrated circuit using a function unit has also been proposed; however, such a differentiator is disadvantageous in that the circuit construction is highly complicated and difficulties are encountered in an attempt to achieve miniaturization.